1. Field of Application
The following description relates generally to telecommunications systems and wireless communications systems.
2. Prior Art
The data throughput of a wireless cellular network is the most important measure of the network performance. The higher the throughput, the more mobile users each cell in the network can serve, and the higher the data rate each user may have. A mobile user may have a very high peak throughput when it is very close to the base station. At cell edge, however, a mobile user may experience extremely low throughput due to interferences from other cells. Since statistically there are many more mobile users in the cell edge area than there are in the proximity of the base station, the average cell throughput can be much lower than the peak throughput. Typically the average cell throughput can be an order of magnitude below the peak throughput, or lower.
Recently, in an effort to improve the network performance, multipoint broadcast or multipoint transmission schemes have been introduced to wireless cellular networks. An example is CoMP, or Coordinated Multi-Point transmission, in an LTE-A (long-term evolution of 3rd generation of wireless cellular network, advanced) network. A multipoint broadcast scheme aims at increasing the cell-edge performance and can be described as follows. Refer to FIG. 1 where an example multipoint broadcast system can be identified. Base stations 112, 114, and 116 form a set of collaborating multipoint broadcasters. Mobile users 122, 124, and 126 form a set of recipients in the multipoint broadcast system. In the multipoint broadcast system in FIG. 1, base stations 112, 114, and 116 transmit the combinations of the signals intended for mobile users 122, 124, and 126. For each base station, the combination “weight” for each mobile user signal can be different. Through elaborate algorithms, the signals are combined at each base station in such a way that when the transmitted signals from base stations 112, 114, and 116 arrive at mobile user 122, the signals for mobile users 124 and 126 are cancelled out or minimized, while the signal for mobile user 122 is maximized or enhanced, thus the signal quality of mobile user 122 improves significantly. Similarly, mobile users 124 and 126 will also see significant improvement in the quality of their respective signals. The combining of the signals at each base station is commonly referred to as “pre-coding”. The combining weights for each mobile-user signal and for each base station constitute the elements in a so called “pre-coding matrix”.
In a cellular network, the channels from base stations to mobile users are referred to as the downlink, and the channels from mobile users to base stations are referred to as the uplink. In multipoint broadcasting, the downlink-channel information is required at the collaborating base stations. In an FDD (frequency-division duplex) network, such information is fed back by the mobile users via the uplink. The feedback overhead can be so large that the uplink capacity can be diminished. A TDD (time-division duplex) network can theoretically eliminate most of the feedback overhead by utilizing the channel reciprocity, i.e, the downlink and uplink channels are identical since the downlink and uplink share the same radio frequency. Thus base stations can obtain the downlink-channel information by estimating the uplink channel.
Channel reciprocity, however, applies only to the wireless channels between the antennas of base stations and mobile users. If base stations and mobile users have different transmitter (TX) and receiver (RX) characteristics, which is typically true, the reciprocity then does not exist in the overall channels between base stations and the mobile users, which includes the wireless channels between base station antennas and mobile user antennas, and TX and RX chains of base stations and mobile users. One approach to overcoming the non-reciprocity is to feed back the complete downlink-channel information via the uplink. This will incur the same feedback overhead as in FDD, but the remaining uplink capacity is even smaller since the total available uplink capacity is already reduced by the downlink traffic that shares the same frequency band with the uplink. Another approach is to “calibrate” the TX/RX differences. The calibration of base stations involves the following steps: (i) mobile users send pilot signals to base stations, (ii) base stations estimate uplink channels with pilot signals from mobile users, (iii) base stations send pilot signals to mobile users, (iv) mobile users estimate the downlink channel with pilot signals from base stations, (v) mobile users feed the complete downlink-channel information back to base stations, and (vi) base stations use the differences between downlink and uplink channels to calibrate the TX/RX mismatches. The calibration of mobile users can be done in a similar fashion.
The calibration approach, however, has several drawbacks that keep it from being a feasible solution. First, the calibration is a slow process due to extensive information feedback and calibration algorithms, which will suffer loss of fidelity under rapidly changing channel conditions. Second, some part of the TX/RX mismatches is fixed or slowly changing, and some part is relatively quick time-varying. The calibration can only compensate the fixed or slow-changing portion of the TX/RX mismatches, thus offers little help in removing the channel non-reciprocity. Third, even if the calibration is made to occur more often to track the quick time-varying portion of the RF mismatches, the feedback overhead will approach to that in FDD, since the standard calibration procedures require feedback of the complete channel information over the entire signal bandwidth, thus defeating the purpose of the calibration.
Without an effective and feasible method to compensate and to track the TX/RX mismatches, the benefits of multipoint broadcasting will be diminished by lack of accurate downlink-channel information and/or by high feedback overhead.